I. Field of the Present Invention
The present invention relates generally to metal making apparatus, and more particularly to apparatus for separating slag from molten metal as the molten metal is transferred from a receptacle.
II. Description of the Prior Art
In metal making processes, the flow of molten metal through the discharge nozzle of a receptacle such as a furnace, tundish or ladle induces a swirl to the molten metal above the discharge nozzle. At a critical level, the energy of the swirl creates a vortex, whereby the slag layer lying on top of the molten metal is sucked into the nozzle and mixes with the molten metal being poured through the nozzle. Several devices have been known to inhibit introduction of the slag to the discharge nozzle in order to avoid the contamination of the poured metal. Before such devices were introduced, it was necessary to terminate pouring by closing the nozzle while the molten metal is at a level above the point at which suction action of the vortex draws slag downwardly into the discharge nozzle. Such a procedure traps a large amount of molten metal in the receptacle and reduces effective yield of the melt.
U.S. Pat. No. 4,494,734 LaBate, et al discloses a dart incorporating a depending guide member engageable in the tap hole to direct accurate placement of the stopper body toward the tap hole. The patent discloses alternate configurations of the stopper body which are taught to cause swirling of the metal. The patent teaches that visual observation of the swirling metal and slag indicates when tapping of the furnace is to be terminated.
Many of the previously known devices for restricting slag from flowing through the discharge nozzle were in the form of plugs which lodge in the discharge nozzle to prevent further pouring through the nozzle. For example, U.S. Pat. No. 2,810,169 to Hofer discloses the use of a slag dam as well as a plug 3 which is mechanically controlled for placement during the pouring operation. However, such units are large and expensive to build, and numerous parts are subjected to the harsh environmental conditions of molten metal. Consequently, the cost of repairing or replacing the parts substantially increases the costs of metal making.
Other known devices for separation of slag and molten metal during discharge comprise bodies which are self-supporting in the molten metal layer. This is accomplished by constructing the body with a specific gravity between the specific gravity of the molten metal and the specific gravity of the slag layer. One such device with such controlled density is a spherical body which is drawn by the vortex into the nozzle and lodges in the nozzle to obstruct further flow. However, such a device is difficult if not impossible to remove, and frequently required replacement of the nozzle sleeve.
U.S. Pat. No. 4,526,349 to Schwer discloses an annular disc having a specific gravity which permits it to separate slag at the interface between the slag and steel. However, this patent also contemplates that a spherical body is drawn by the suction of the vortex into the discharge nozzle opening to cut off fluid flow. The patent discloses that two discrete articles are necessary. While the ring is taught to counter the effect of the vortex formed over the discharge nozzle, the sphere merely closes off pouring when slag is about to be introduced to the discharge nozzle. Consequently, the problems previously discussed are encountered.
A self-supporting device particularly developed to inhibit the vortex formation was disclosed in the present applicant's previous U.S. Pat. No. 4,601,415. That patent defined a tapered, polygonal body designed to generally conform with the shape of the vortex along its length so as to extract energy from swirling movement of the molten metal. Unlike previously known plug bodies, the patented vortex inhibitor was taught to be self-orienting by its vortex conforming shape. However, to assure apex downwardly positioning, the patent also disclosed a weighting means embedded in the refractory body. Moreover, while the patent teaches that the shape of a polygonal cross-section may be changed to adjust the degree of plugging or the throttling effect as the body enters the discharge nozzle, it was found that changing the shape of the vortex inhibitor from the tetrahedral shape of the preferred embodiment could affect the orientation stability of the body. In particular, as the body is subjected to outside influences during placement in the molten metal bath or during movement of the receptacle from which the molten metal is being discharged, the geometrical proportion can disrupt the apex downward orientation desired. Moreover, the use of a separate weighting means, such as a core, substantially increases the complexity of producing the vortex inhibitor and has been disfavored.